I have read a great deal of books and academic sites on
Quantum Mechanics and find it very interesting, particularly where
Quantum Entanglement is concerned.
The problem I have with entanglement is that it all seems so simply
explained in other ways which do not require 'action at a distance'
What I would like to know is whether there is any experimental data that
either proves or strongly suggests that the state of each 'entangled'
particle was not already defined from the outset.
I read an account of a box which emits red light from one side and blue
light from the other. These colors are randomly switched. An observer
measuring the light from one side can then 'know' the color that a second
observer will measure from the other side - Even though this reduces the
probability of the second observer getting a particular colour from 0.5 to
a definite 1 or 0 it does not mean that any information has been
transmitted. Similarly, if I toss a coin and measure the UP side for
'heads' or 'tails' this does not affect or in any way alter a later
observation of the DOWN side.
I am sure all these physics theorists are not getting so fired up about
something so simple - so I must assume that I am missing some experimental
evidence somewhere. Can you point me to the paper or explanation that
will help me see this subject as something more than mundane ?
I recently heard of a successful experiment in Quantum Cryptography which
I had assumed would demonstrate the ability to transfer information via
entanglement. What I found was a disappointment since it relies on a
second channel of regular communication and entanglement simply allows the
sender to know what was sent.
The only real advantage of Quantum Cryptography is the current practical
difficulties in reading a particles state without significantly altering
it. Being a new science I do not find this current limitation
particularly assuring ... far less so, in my view, than conventional
I am not normally such a cynic, honest. As I said before - I am pretty
sure I am missing something important here. Can you help me?
No, Monica, there is quantum weirdness. You are not missing something. I
am not a theoretical physicist, nor do I presume to really understand the
phenomenon. However, it is possible to read the experiments done on
entanglement and come to the results of those experiments. The results are
"facts" they cannot be disputed. Where the paradox emerges is when we try
to "explain" the results verbally in terms of pictures and words that
really do not apply. It may be, and probably is, the case that we do not
have the proper analogs to "explain" the results. In this case what the
theoretician has to do is make sure the math is correct and abandon the
physical explanation in terms of models that may not be appropriate.
Entanglement is a quantum phenomenon, and trying to "explain" the results of
the mathematics in terms of macroscopic springs, and balls and stuff that we
are familiar with is just not appropriate. All that can be done is let the
math take us where it will (assuming it is done correctly). It is a general
result of differential equations that if there are two valid solutions to
the equation, then any linear combination of those solutions will also be a
valid solution. In the case of entanglement what this "means" is that a
photon (electron or whatever) that passes through two slits, it actually
passes through both slits simultaneously, no matter how far apart the two
slits are arranged. There is no classical analog to that behavior.
In addition to the book "Entanglement: The Greatest Mystery in Physics" by
Amir D. Aczel, the web sites below discuss entanglement. However, the
bottom line is the "explanations" are not very convincing in classical terms.
Quantum mechanics gives the correct description of the experimental
results. The fact that it does not make sense in classical terms is just
the limitation of our experience.
Clarification Point January 2006
Vince Calder's answer to Monica's question (#1770) about quantum
entanglement misses the point of her question. Clearly she believes that
the experiments show nothing weird at all, and that the physicists appear
to be all excited about nothing. His answer that "she is not missing
something" and "there is quantum weirdness" contradict each other since
she thinks nothing weird is going on.
For example, the EPR paradox about the quantum entanglement of the spin of
two electrons DOES seem mundane to an outsider. After all, if you just
assume that each electron has an inherent spin that is equal and opposite
to the other one, then measuring the spin of one tells you the spin of
both electrons. She wants to know, "what is the big deal?"
The accepted quantum explanation is that the electrons' spins were in a
superposition of states and that there is a collapse of the wave function
which results in an instantaneous change to the wave function everywhere
as soon as one of the spins is measured. Indeed, Einstein's whole point
with the EPR paradox was that it is more reasonable to believe that the
electrons DO already have the appropriate spin quality than to believe in
an instantaneous collapse of the wave function - something he called
*spooky action at a distance* and which he thought must violate
relativity. The fact that quantum mechanics successfully predicts the
answer is great. But Monica's question seems to be, "how do you know
there is not a simpler explanation, such as, the electrons always had the
appropriate inherent spin in the first place (like Einstein believed) so
that nothing *weird* is happening at all?"
I suspect the answer she is looking for has something to do with Von
Neumann's proof that you cannot supplement quantum mechanics with hidden
variables in such a way as to make all of the observables "dispersion
free". Bell's theorem would also probably be relevant reading.
I do not want to get into a game about quantum weirdness, EPR paradox, or
action at a distance. First, it severely challenges my competence in the
area. Second, the concept of "entanglement" is mathematically abstract and
complicated. I would ask these questions of the responder above: 1. What
does the term "an instantaneous change to the wave function" mean? The wave
function is a continuous function of time, and consequently it cannot have
"an instantaneous change" which implies a discontinuous function. 2. What
specifically and mathematically is meant by "a collapse of the
wavefunction?". 3. The information of measuring the spin of an electron at
point "A" cannot be transmitted to point "B" faster than the speed of
light, so the second electron does not "know" whether it is 'up' or 'down'
relative to "A", before the measurement of "A" even occurs. 3. The term
"dispersion free" is not defined by the responder. 4. I refer the responder
to chapter 7 of the book "The Odd Quantum" by Sam Treiman, and still
maintain that there is not something weird that is not well understood.
I defer to a quote of Richard Feynman to the effect that anyone who
claims to understand quantum mechanics, does not understand the problem.
Click here to return to the Physics Archives
Update: June 2012